The present invention relates to the digital image processing arts. It is particularly related to processing of binary halftone images for purposes of controlling darkness/lightness levels. Binary halftone images are often xe2x80x9cblack and whitexe2x80x9d images, and the present invention is described with reference thereto for ease of understanding. However, it is not intended that the invention be limited to black-and-white images, but multi-separation color images are explicitly included.
To lighten or darken a digital image defined by a plurality of pixels expressed in terms of a gray scale, the input data is subjected to a tone reproduction curve (TRC) that maps the input pixel gray values to lighter/darker values as desired. However, for binary images, such a simple operation is not possible given that each pixel is defined as one of two different possible values.
A large amount of xe2x80x9cprint-readyxe2x80x9d binary halftone images exist in image archives as legacy documents or in xe2x80x9cfar away memoryxe2x80x9d such as fax machines and remote copy print. These binary halftone images are defined by a bitmap or array of pixels having xe2x80x9cmxe2x80x9d columns and xe2x80x9cnxe2x80x9d rows. Each pixel is binary, i.e., it is defined in one of two possible states such as, for example: (i) xe2x80x9conxe2x80x9d or xe2x80x9c1xe2x80x9d to indicate the presence of a first state such as white; and, (ii) xe2x80x9coffxe2x80x9d or xe2x80x9c0xe2x80x9d to indicate a second state such as black.
In a halftone image, an adjacent group or cell of binary pixels, referred to as a halftone dot, are used to simulate gray levels between the two binary states. The gray level of the cell is varied by controlling the number of on/off pixels in the cell. The human visual system possesses a limited frequency response and, to a good approximation, spatially integrates the binary data within the cell to derive an overall gray level of the cell.
It is desirable to be able to process binary halftone images to at least some degree to enhance their appearance. For example, it is highly desirable to be able to lighten or darken a binary halftone image. Unfortunately, virtually all image processing is xe2x80x9clockedxe2x80x9d in the data.
The technique disclosed in commonly assigned U.S. Pat. No. 5,258,854 to Eschbach allows for darkness/lightness control of binary halftone images. While this prior method is highly effective, it relies upon an output data resolution that is higher than the input data resolution during at least the processing of the data. Therefore, it is generally more complex and time-consuming to implement and, thus, not suited for all image processing platforms or environments.
Coarse adjustment of darkness and lightness can be achieved simply by directly adding white or black pixels to the image data as desired. For example, to darken and image, additional black pixels can be added using morphological operations. To lighten and image, white pixels are added. This results in very coarse lightness/darkness adjustment and generally leads to sub-optimal results.
In light of the foregoing a need has been identified for a digital image processing method and apparatus for controlling lightness and darkness in a binary halftone image in an efficient manner without degrading image quality.
In accordance with the present invention, a novel and unobvious method and apparatus are provided for controlling lightness/darkness in binary halftone digital images
In accordance with one aspect of the invention a method for adjusting the lightness/darkness of a binary halftone image includes receiving digital image data defining a binary halftone image in terms of a plurality of light pixels having a first value and a plurality of dark pixels having a second value. If the binary halftone image is to be lightened, adjusting the first value in a direction away from the second value by a select amount for each of the plurality of light pixels. If the binary halftone image is to be darkened, adjusting the second value in a direction away from the first value by a select amount for each of the plurality of dark pixels. An error diffusion operation is then performed on the binary halftone image.
One advantage of the present invention resides in the provision of a method and apparatus for controlling lightness/darkness in binary halftone digital images wherein only strict pointwise operations and error diffusion type operations are performed so that the bit depth and resolution of the input image equals the bit depth and resolution of the output image.
Another advantage of the present invention is found in the provision of a method and apparatus for controlling lightness/darkness in binary halftone digital images wherein halftone dot spacing and position are maintained.
A further advantage of the present invention resides in the provision of a method and apparatus for controlling lightness/darkness in binary halftone digital images wherein processing error is controlled to prevent instabilities in the resulting output image.
Still other benefits and advantages of the present invention will become apparent to those of ordinary skill in the art to which the invention pertains upon reading and understanding this specification.